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There were two problems here: the QDateTime comparisons didn't return true for files with unchanged mtime. Using the same conversion on both sides of the comparison fixes that. As a result, startup now becomes much faster since we're not rebuilding the cache each time. Since the format of the date has changed on-disk, this should be backwards compatible, causing a one-time rebuild. That exposed another problem that I was half-observing already before, which is that the PNG thumbnails weren't being restored properly. Converting the raw data to base64 when saving fixes that as well. (cherry picked from commit 6fc25040)
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